Imidazole type curing agents

ABSTRACT

The disclosed curing agents or initiators have the formula ML n  (O--SO 2  --R f ) m  wherein L is an imidazole, M is a metal, n is a coordination number of M, R f  is a fluorinated alkyl group, and m is the valence of M. These curing agents are useful in latent curable epoxy systems. Upon heating to the cure temperature (e.g. 100° - 250° C.), such systems cure efficiently and with a low exotherm.

This invention relates to coordination compounds which are useful aslatent initiators (i.e. curing, hardening or activating agents). Anaspect of this invention relates to low-exotherm curing of strainedheterocyclic systems such as epoxides (oxirane ring-containingcompounds) or the like. A further aspect of this invention relates to aone-part system comprising a curable epoxide and a fluoroalkylsulfonatesalt of a metal coordinated with imidazole or substituted imidazoleligands.

It is well-known that the oxirane or 1,2-epoxy group, similar strainedheterocyclic groups, and similar polymerizable or crosslinkable systemsare subject to attack by a proton or by the unbonded electron pair of anamino nitrogen. Both imidazole and its substituted derivatives(hereinafter collectively referred to as "imidazoles" or as "theimidazole nucleus") have been investigated as initiators, e.g as curingagents for epoxy resins, and have been found to produce cured (i.e.polymerized and/or crosslinked) resins with good properties; see forexample, Paul F. Bruins, Ed., Epoxy Resin Technology, IntersciencePublishers, N.Y. (1968) and Farkas et al, J. Applied Polym. Sci., 12,159 (1968). Unless the 1-position of the imidazole nucleus is eitherblocked in some manner (e.g. see U.S. Pat. No. 3,356,645 to Warren,issued Dec. 5, 1967) or quaternized, it contains a secondary(1-unsubstituted) or tertiary (1-mono-substituted) nitrogen with anunbonded electron pair. This electron pair is ordinarily quite reactivewith the 1,2-epoxy group even at relatively low ambient temperatures;therefore, imidazoles are not considered "latent" curing agents orinitiators. In this art, a "latent" curing agent or initiator is onewhich does not react readily with the oxirane ring (or similar strainedheterocycles) at ordinary ambient temperatures, but which can be made toreact readily under certain specific conditions, e.g. elevatedtemperatures. Typically, such elevated temperatures are above 50° C.and, in industrial practice, above 100° C. A latent curing agent orinitiator can therefor be included in a one-part curable system which isstorable (i.e. will remain substantially free of gelling or hardeningdue to polymerization or crosslinking) for long periods of time. Statedanother way, the pot life of a curable resin/"latent" curing agentmixture is extremely long unless the mixture is heated.

German Offenlegungsschrift (DOS) No. 1,904,641 (laid-open date, Nov. 6,1969) discloses a class of curing agents obtained by reacting imidazoleswith certain metal salts; the resulting products are referred to as"metal salt complex compounds of imidazoles". Theoretical studies reportthat the imidazole nucleus can be a ligand for metals of Groups VIII,IB, II (A and B), and VII B. Such studies are evidence for theproposition that the cationic portion of the metal-imidazole salt"complex compounds" described in DOS 1,904,641 consists of a metalliccation combined with an appropriate (coordination) number of imidazoleligands. See, for example, J. Am. Chem. Soc. 76, 3054 and 6219 (1954),77, 859 and 5291 (1955), 78, 260 (1956), and 80, 5033 (1958); J. Chem.Soc. 1961, 4790, (A) 1967, 757 and (A) 1969 368. This proposition is notentirely supported by the disclosure of DOS 1,904,641 in view of theteaching on page 5 that the metal salt/imidazole molar ratio is "notcritical" and can be between 1:1 and 1:6; the literature establishesthat the number of moles of imidazole is a small integer greater than 1,this integer being fixed by the various possible coordination number ornumbers of the metal, e.g. 2, 3, 4, or 6. The making of a metalsalt/imidazole coordination compound which consists of an equivalent ofmetal and a single mole or fractional number of moles of imidazole hasnever been reported and would be contrary to established principles ofcoordination chemistry. Apparently the aforementioned DOS teachingmerely describes the amount of imidazole starting material to be usedand does not represent an analysis of the "complex".

The initiation reaction, which activates a system containing a strainedheterocycle and leads to gelling or curing (hardening) due topolymerization, crosslinking, etc. is wellknown. For imidazole-typeinitiators (including prior art types and the initiators of thisinvention), the 1-nitrogen of the imidazole nucleus or moiety reactswith the strained heterocyclic ring according to the following equation:##STR1## In thisequation compound (I) contains at least one strainedheterocyclic ring, Q being, for example, oxygen or N-Y, wherein Y is anorganic radical. Compound II is an imidazole, only the 1-position beingshown, and the R substituent is hydrogen or an organic radical such as alower alkyl group. Compound (III) is the product of the initiationreaction, i.e. the material obtained by cleavage of the heterocyclicring. See Lee et al, Handbook of Epoxy Resins, N.Y., McGraw-Hill, 1967,Chapter 5, pp. 5-1 and 5-2. Further reactions can, of course, occurafter compound (III) is formed, e.g. polymerization of compound (I) orcrosslinking of polymer chains if compound (I) is already a polymer orprepolymer.

The reaction represented by the above equation is strongly exothermic.The exotherm of a curable epoxy compound is defined as "the increase intemperature of the compound above the cure temperature due to energiesreleased as the epoxy groups react." Lee et al, op. cit., Chapter 17,page 17-8. The higher the exotherm, the more likely that detrimentaleffects will be observed during curing, e.g. gas formation, explosions,uneven gelling, shrinkage, and charming or similar thermal degradation.Lee et al, op. cit., pp. 17-9 and 17-10. These detrimental effects aremost commonly encountered with massive or relatively thick, as opposedto "film- or layer-like" castings, i.e. castings with geometricallysolid shapes having a significant third dimension or a highvolume-to-area ratio. Rapid curing of a typical epoxy resin castingusing the latent curing agents of DOS 1,904,641 or similar latent curingagents requires heating the casting to a temperature referred to by Leeet al as the "cure temperature". This temperature is not necessarily theonly temperature at which hardening due to crosslinking orpolymerization or the like will be initiated. Rather, it is thetemperature, or range of temperatures, at which complete curing to ahard material is efficient and rapid and produces a cured (polymerizedand/or crosslinked) material with good properties. Typically, an epoxyresin casting is heated quite slowly to the "cure temperature", which isordinarily somewhere in the range of 100° - 200° C. and generally atleast 120° C., in order to avoid the detrimental effects of a highexotherm. For a three dimensional casting (e.g. a 20 mm [diameter] × 30mm [length] cylinder) comprising 10 g. of a typical one-part epoxymaterial such as the polyglycidyl ether of bisphenol A andepichlorohydrin and an appropriate amount of a preferred prior artmetal-imidazole "complex" such as nickel or copper chloride-imidazole,the center of even this small casting would reach a peak exothermtemperature at least 130° C. above the "cure temperature", if thecasting were heated to the "cure temperature" in a few minutes or less,e.g. by immersion in a preheated bath. Such peak temperatures are, inmany instances, high enough to cause at least some of the detrimentaleffects described by Lee et al. Thus, the use of such prior art curingagents involves slowly heating the massive casting to the curetemperature. This slow heating is economically wasteful and not alwayseffective in mitigating degradation cause by high exotherms. A furtherproblem encountered in the application of these prior art curing agentsis that many of these agents will not cure cycloaliphatic epoxides.

Accordingly, this invention contemplates initiators or curing agents ofthe metal salt-imidazole type wherein the exotherm caused by the openingof a strained heterocyclic ring is adequately controlled, i.e. is keptwithin non-degradative limits.

This invention also contemplates a curable epoxy system containing alatent curing agent or initiator, the system having a curing temperaturewhich is preferably above 100° C., wherein the peak exotherm temperaturereached during curing is significantly below the temperature at whichundesirable side effects (e.g. charring) occur, regardless of howquickly the curable epoxy system is brought to the cure temperature.

Briefly, this invention involves the use of a particular class of metalsalts which react with imidazole nuclei to form latent initiators orcuring agents which efficiently and controllably initiate ring-openingreactions, e.g. the curing of epoxy compounds (including cycloaliphaticepoxides) at temperatures above 50° C. without causing unduly high peakexotherm temperatures.

The salts of this class (which are known per se or can readily beobtained from the corresponding fluoroalkyl-sulfonic acids, see U.S.Pat. No. 2,732,398 to Brice et al, issued Jan. 24, 1956) can berepresented by the formula:

    M(O--SO.sub.2 --R.sub.f).sub.m                             (IV)

in formula (IV), M is a metal of groups VIII, IB, II(A and B), IIIA,IV(A and B), VIB, or VIIB of the Periodic Table. The transition metals(metals of the "B" Groups and Group VIII) are preferred, particularlyGroup VIII and Groups IB and IIB, which are especially suitable in thecontext of this invention. The metals of the first triad of Group VIII(Fe, Ni, Co) and copper, cadmium, and zinc are quite desirable from aneconomic standpoint. Silver is economical as compared to either gold orthe noble metals of Group VIII.

The term R_(f) designates a C₁ - C₁₈ fluorinated alkyl group wherein thecarbon atom alpha to the sulfonyl radical has its other three valencebonds taken up by fluorine atoms or fluorinated alkyl substituents.R_(f) is preferably a perfluorinated straight or branched-chain orcyclic alkyl group of 1-18 (preferably 1-8) carbon atoms, e.g. CF₃, C₂F₅, n-C₃ F₇, i-C₃ F₇, n-C₈ F₁₇, perfluorocyclohexyl, perfluoro(4-methylcyclohexyl), 2-perfluorocyclohexyl perfluoroethyl, etc.

Suitable less highly fluorinated R_(f) radicals include HCF₂ CF₂, C₃ F₇CHFCF₂, (CF₃)₂ CHCF₂, CF₃ -CFCl-CF₂, ClCF₂ CF₂, and the like. In no caseshould a hydrogen substituent be located on the carbon alpha to thesulfonyl.

The term m is a number from 1-4, preferably from 1 to 3. Thus, thepreferred salts contain iron (II), iron (III), Co(II), Co(III), Ni(II),copper (I), copper (II), cadmium, zinc or silver cations and thecorresponding stoichiometric number of perfluoroalkylsulfonate anions.

The salts of formula (IV) are reacted with a suitable imidazole (i.e.imidazole or a substituted imidazole) to provide the latent curingagents of this invention, which agents can be represented by thefollowing formula:

    ML.sub.n (O--SO.sub.2 R.sub.f).sub.m                       (V)

wherein L is the ligand (the imidazole or substituted imidazole ) n is awhole number ranging from 2-12 (preferably 2-8), and R_(f) and m are asdefined previously.

Suitable imidazoles include imidazole itself (NH--CH═N--CH═CH) or aderivative thereof substituted at the 1, 2, 4, and/or 5 positions (the1-position is the secondary or "pyrrole" nitrogen, ═NH, and the3-position is the "pyridine" nitrogen, ═N-), but the 1-substitutedimidazoles tend to be very slow to react, even at the cure temperature,and are not preferred except for applications where very slow cures aredesirable. Thus, it is preferred that L have the following formula:##STR2## wherein R₁, R₂, and R₃ are the same or different and can behydrogen, halogen, or an organic radical such as a hydrocarbon orsubstituted hydrocarbon radical, e.g. alkyl (preferably lower alkylincluding aryl-substituted alkyl), aryl (preferably monocyclic aryl suchas phenyl), aralkyl, alkenyl (e.g. vinyl, allyl), and other substituentsknown in the art (see, for example, the aforementioned U.S. Pat. No.3,356,645 and DOS 1,904,641) and R₁ and R₂ can together comprise theatoms of a fused ring such as the three or four atoms of a cyclopenteneor cyclohexene ring, a benzene ring or the like, and R₁ or R₂ togetherwith R₃ can comprise the atoms of a fused N-heterocyclic ring. Forefficient curing at the curing temperature, it is preferred that R₄ behydrogen. If the R₄ substituent is not hydrogen, it can be similar toR₁, R₂, or R₃. Examples of preferred compounds of Formula (VI) areimidazole; 2-methyl imidazole; 2-ethyl imidazole; 2-ethyl, 4-methylimidazole; 2,4-dimethyl imidazole, 2,4-diethyl imidazole;2,4,5-trimethyl imidazole; 2-benzyl imidazole; 2-benzyl, 4-methylimidazole; and 2-phenyl imidazole. The 1-substituted imidazoles are, ashas been pointed out, also operative, and examples of these are 1-methylimidazole, 1,2-dimethyl imidazole, and 1-phenyl, 2-methyl imidazole.

The compounds of Formula (V), which are not reported in the literature,are obtained by mixing a salt of Formula (IV) with an excess overstoichiometry of an imidazole of formula (VI) and subjecting the mixtureto mild heat. The resulting product can be purified by extracting theexcess imidazole or imidazole derivative using conventional extractiontechniques and a suitable solvent such as toluene. The "stoichiometric"amount of the imidazole of Formula (VI) is determined by thecoordination number of the coordination compound obtainable from theparticular salt. If more than one coordination number is known for agiven metal ion (e.g., 2 and 4 in the case of cupric ion), little or noexcess over stoichiometry of ligand-forming material is used in makingthe lower coordination number (n = 2) species, while considerable excesscan be used for the n = 4 species. In an alternative method, higher orlower numbers of ligands can be derived from previously prepared andisolated coordination compounds by adding or driving off ligands, e.g.converting the n = 4 species to the n = 2 species by carefully drivingoff two moles of the imidazole from a mole of the n = 4 species.

Examples of coordination compounds of formula (V) made for use in thisinvention are:

CuL₄ (SO₃ CF₃)₂

CuL₂ (SO₃ CF₃)₂

CuL₃ (SO₃ CF₃)

CuL₄ (SO₃ CF₂ CF₃)₂

CoL₆ (SO₃ CF₃)₂

CoL₆ (SO₃ CF₂ CF₃)₂

NiL₆ (SO₃ CF₃)₂

NiL₆ (SO₃ CF₂ CF₃)₂

NiL₆ (SO₃ CF₂ CF₂ CF₃)₂

CdL₂ (SO₃ CF₃)₂

ZnL₆ (SO₃ CF₃)₂

wherein L is as defined previously in formula (V).

Mixtures of such coordination compounds with themselves or withconventional curing agents can be used to provide a wide variety ofcuring capabilities and to vary the properties of the cured materials.An extensive discussion of mixtures containing conventional curingagents can be found in the aforementioned DOS 1,904,641.

The compounds of formula (V) can best be described as latent"initiators" in that, upon dissociation, they are particularly effectivein initiating ring opening reactions or the like which lead topolymerization and/or crosslinking of strained heterocycle-containingcompounds. The most useful result of such reactions is gelling orsolidification of liquid systems. Liquid, curable epoxy systems can thusbe "cured" or hardened to solids having a Barcol hardness of, forexample, at least 80. In short, the initiators of this invention areparticularly useful as curing agents (also known as "hardeners" or"activators"). The properties of the resulting cured systems are thesame as or similar to the properties obtained from a low temperaturecure with free imidazole.

The peak exotherm temperature produced when compounds of Formula (V) areused as curing agents -- particularly in commercially significantcurable epoxy systems such as the monomeric aliphatic epoxides, thecurable glycidyl ethers and polyglycidyl ethers, the aliphatic epoxidesmodified with glycols, the glycidyl ethers of novolak-type resins, andthe like -- tends to be significantly lower (e.g. more than 50° C.lower) than peak temperatures resulting during the curing of these sameepoxy systems with the prior art metal salt-imidazoles, particularlywhen these epoxy systems are quickly brought to their cure temperatures.Indeed, it is a feature of this invention that suitable curable systemscontaining the initiators of this invention can be controllably curedusing a "hot entry" technique, wherein the curable system is plungedinto a preheated bath or atmosphere and thereby brought to the curetemperature almost instantaneously (in substantially less than 5minutes, for example).

This lowering of the peak exotherm temperatures is less dramatic and mayeven be negligible during the cure of cycloaliphatic epoxides. However,in contrast to the latent initiators or curing agents of this invention,many of the preferred prior art metal salt-imidazole latent curingagents, e.g. imidazole-copper (II) chloride and imidazole-cobalt (II)chloride, appear to be inoperative as curing agents for cycloaliphaticepoxides.

There is no simple theoretical explanation for these differences betweenthe present invention and the prior art, and this invention is not, inany event, dependent upon any theoretical explanation. Thenoncoordinated metal salts per se, i.e. the metal fluoroalkylsulfonates,can be used as latent curing agents for epoxides and the like accordingto the teachings of J. E. Kropp, U.S. patent application Ser. No.813,758, filed Apr. 4, 1969. However, such metal salts, e.g. Co (II), Cu(II), and Ni (II) trifluoromethane sulfonate produce high peak exothermtemperatures; e.g., for a system containing a gram of one of these metalsalts mixed with ten grams of epoxy resin, the peak exotherm is morethan 150° C. higher than the "hot entry" bath temperature of 140° - 160°C. It is not understood why the combination of two catalytic species(the imidazole nucleus and the fluoroalkyl sulfonate anion) in acoordination compound produces improved effects such as better controlover the exotherm during curing. The literature (including theaforementioned DOS reference) does not suggest this improvement. The DOSdoes not teach that the choice of anion is critical; still less does itteach the use of salts containing fluorinated alkyl sulfonate anions,although the structurally analogous trifluoroacetate salts aresuggested. Surprisingly, in terms of peak exotherm temperatures duringcuring of epoxy resins, metal-imidazole trifluoroacetates are notanalogous to the corresponding metal-imidazole trifluoromethanesulfonates.

Although this invention is not bound by any theory, studies of thethermal decomposition of metal-salt-imidazole initiators of thisinvention indicate that, at elevated temperatures, the imidazole orsubstituted imidazole ligand is fully liberated and de-latentized, thuspermitting both the imidazole nucleus and the fluoroalkylsulfonate anionto participate, in some manner, in the initiation reaction.

A further feature of this invention is that the increased control overthe exotherms produced during the initiation and subsequent reactions(which reactions provide useful gelation and/or curing effects) isobtained without unduly slowing down these reactions, provided suitableelevated temperatures are used. Thus, while the gel time of an epoxyresin/curing agent system of this invention can be as long as 4 or 5hours at 100° C., it can be as short as 30 seconds at 200° C. (Gellingor curing initiated at less than 90° C. tends to be far too slow formust uses, and gelling or curing may be uncontrollable at temperaturesabove 250° C.). In the preferred cure temperature range of 120° - 170°C., complete curing is ordinarily obtained in about 1 - 20 minutes (e.g.about 5 - 15 minutes), and generally speaking the cure time is about thesame or slightly longer than cure times of prior art latent curingagents. Since "hot entry" curing can be adequately controlled accordingto the teachings of this invention, the total time involved in thecuring of epoxy systems or the like can be substantially shortened byeliminating or speeding up the slow preheating or de-latentizing cyclenecessitated by prior art latent curable epoxy systems.

Thus, the latency of the initiators or curing agents of this inventionis apparent even at ambient temperatures as high as 90° C. At 90° C. orlower, the initiation reaction or reactions are not observed for severalminutes or hours. At temperatures below 50° C., this period of latencycan be indefinitely long. Liquid curable epoxy systems of this inventionhave been stored at normal ambient temperatures for more than one year,and no traces of gelation have been observed.

For maximum curing efficiency, the preferred curable and gelable liquidsystems of this invention contain more than 0.2 parts by weight ofinitiator per 100 parts by weight (0.2 phr) of the heterocycliccompound, e.g. a curable epoxy resin. Less than 0.2 phr can be used ifsmall amounts of additional curing agents such as dicyandiamide arecombined with the metal fluoroalkylsulfonate-imidazole initiator. Theupper limit of initiator concentration is fixed more by economic than bytheoretical considerations. Thus, amounts up to 25 phr by weight ofinitiator can be used, but are not necessary, since ordinarily amountsof less than 15 phr by weight will provide effective initiation of aring opening reaction or the like. It is significant that, in any event,the ratio of equivalents of activatible imidazole to equivalents ofreactive heterocycle (e.g. of epoxy) can be substantially less than 1:1and generally less than 0.3:1, indicating that an effect in the natureof catalysis is obtained.

In a ten gram sample cylindrical casting of a one-part curable epoxysystem of this invention, the casting being about 20 mm in diameter and30 mm in length, a "hot entry" cure initiated at 140-160° C. and acuring agent concentration of less than 5 wt. % produces a peaktemperature during the cure which is well below 275° C. and generallybelow 225° C. In curing a similar sample casting comprising thepolyglycidyl ether of bisphenol A and epichlorohydrin and 5 wt. percentor even 10 wt. percent of an initiator of this invention, peaktemperatures as low as 203° C. have been observed. With sample castingscontaining cycloaliphatic epoxy as the curable material, the peaktemperatures observed during curing are generally lower, e.g. 190°-210°C.

The systems which are most readily and controllably attacked by thelatent initiators of this invention are compounds, preferably polymers(including dimers and trimers, copolymers, prepolymers, etc.) containingan average of at least one (preferably at least two) strainedheterocyclic rings per average molecular weight and an equivalent weight(per heterocycle) of at least about 100, though lower equivalent weightmaterials are also operative. Theoretically, saturated rings of lessthan 5 members are under strain and are substantially inflexible, whilesaturated rings of greater than 6 members are under strain but areflexible, permitting at least some relief of strain, six-member ringsbeing strain-free in the "chair" or "boat" configuration. Mostcommercially significant are the three-member rings wherein thehetero-atom contains an unbonded pair of electrons, e.g. a 3-memberO-heterocycle (i.e. the 1,2-epoxy or oxirane group) or N-heterocycle.

Compounds containing the 1,2-epoxy group are also known as epoxides andcan be mono- or polyepoxides. Curable one-part epoxy systems can beobtained according to this invention by simply mixing together theinitiator of this invention (with or without additional curing agents orinitiators) and one or more of these mono- or polyepoxides at ambienttemperatures. If desired, a conventional organic solvent can be used tofacilitate mixing. Admixture of the initiator and a conventional curableepoxy resin imparts a color to the system prior to curing. Upon reachingthe cured state, the conventional amber or brown color is observed. Theuncured system is insensitive to water, hence the results of initiatingthe curing reactions do not vary due to atmospheric humidity.

Epoxides suitable for use in this invention can be aliphatic,cycloaliphatic, aromatic or heterocyclic and will typically have anepoxy equivalency (i.e. the number of epoxy groups contained in theaverage molecule) of from 2.0 to 6.0 preferably 2 or 3, this value beingthe average molecular weight of the epoxide divided by the epoxideequivalent weight. The epoxy equivalent weight, which is determined bymultiplying the sample weight by 16 and dividing by the number of gramsof oxirane oxygen in the sample, is typically greater than 100 forcommercially useful curable systems. The lower the equivalent weight,the higher the exotherm for a given sample weight. Typical of suchepoxides are the glycidyl-type epoxy resins, e.g. the diglycidyl ethersof polyhydric phenols and of novolak resins, such as described in"Handbook of Epoxy Resins", by Lee and Neville, McGraw-Hill Book Co.,New York (1967).

Particularly useful epoxides which can be used in this invention arethose which contain one or more cycloaliphatic epoxide groups such asthe epoxycyclohexanecarboxylates, typified by 3,4-epoxycyclohexylmethyl3,4-epoxycyclohexanecarboxylate, 3,4-epoxy-2-methylcyclohexylmethyl3,4-epoxy-2-methylcyclohexanecarboxylate, andbis(3,4-epoxy-6-methylcyclohexylmethyl) adipate. For a more detailedlist of cycloaliphatic epoxides of this nature, reference is made toU.S. Pat. No. 3,117,009.

Further epoxides which are useful in the practice of this inventioninclude glycidyl ether monomers of the formula ##STR3## where R is alkylor aryl and n is an integer of 1 to 6. An example is the glycidyl ethersof polyhydric phenols obtained by reacting a polyhydric phenol with anexcess of chlorohydrin, such as epichlorohydrin, e.g. the diglycidylether of Bisphenol-A. Further examples of epoxides of this type whichcan be used in the practice of this invention are described in U.S. Pat.No. 3,018,262.

Still further examples of epoxides are disclosed in DOS 1,904,641, citedpreviously.

The N-heterocyclic materials suitable for use in this invention includethose having the structures ##STR4##

There are a host of commercially available epoxides which can be used inthis invention, including the diglycidyl ether of Bisphenol A (e.g.,Epon 828 and DER 332), vinylcyclohexene dioxide (e.g., ERL-4206),3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (e.g.,ERL-4221),3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate (e.g. ERL-4201),bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate (e.g., ERL-4289),bis(2,3-epoxycyclopentyl)ether (e.g. ERL-0400), aliphatic epoxy modifiedwith polypropylene glycol (e.g., ERL-4050 and ERL-4052), dipentenedioxide (e.g. ERL-4269), epoxidized polybutadiene (e.g., Oxiron 2001),silicone epoxy (e.g., Syl-Kem 90), 1,4-butanediol diglycidyl ether(e.g., Araldite RD-2), polyglycidyl ether of phenolformaldehyde novolak(e.g., DEN-431 and DEN-438) resorcinol diglycidyl ether (e.g.,Kopoxite), and epoxidized unsaturated glyceryl esters of carboxylicacids having more than six carbon atoms, e.g. epoxidized soybean oil.Low equivalent weight acetals such as trioxane and low equivalent weightepoxide monomers such as propylene oxide, the epihalohydrins, glycidol,etc., are also operative in this invention, bearing in mind thedifficulty of controlling the relatively high exotherm (due to the lowequivalent weight).

Any of the conventional filler materials can be added to the curablesystems of this invention including pigments and the like. Curablesystems of this invention can be used in the conventional manner to makecoated, impregnated, or molded products, e.g. structural panels and thelike. The low exotherm evolved by systems of this invention isparticularly advantageous in the making of molded products withrelatively thick or massive shapes, i.e. as opposed to film orlayer-like objects.

In the following non-limiting Examples, all parts are by weight unlessotherwise indicated.

EXAMPLE 1 Preparation of [Zn(C₃ N₂ H₄)₄ ](SO₃ CF₃)₂

Zinc (II) perfluoromethane sulfonate (20 gm - 0.04 mol) was mixed withsolid imidazole (15 gm - 0.22 mol) in a 150 ml. beaker on a hot plateand the mixture was melted. After cooling the product, it was placed inthe thimble of a soxhlet extractor and the excess imidazole wasextracted by conventional soxhlet extraction using toluene as a solvent.

Calculated: 10.3% Zn

Found: 10.3% Zn

EXAMPLES 2 - 4 Preparation of Co(II), Ni(II), andCu(II)-Imidazole-Perfluoromethane Sulfonate

The procedure of Example 1 was followed in substance, except that 0.11mol of Co(II), Ni(II), or Cu(II) perfluoromethane sulfonate wasdissolved in 300 ml. methanol and reacted with 0.7 mol (in the case ofthe cobalt and nickel salts) or 0.5 mol (in the case of the copper salt)of imidazole. The solution was concentrated by evaporation to 150 ml.Addition of 200 ml. of ether precipitates the desired salt. The salt wasfiltered and washed with ether and dried at room temperature. Thefollowing products were obtained: (L = imidazole)

    ______________________________________                                             Product                                                                  Ex.  (L=C.sub.3 N.sub.2 H.sub.4)                                                               % Metal Calc.                                                                             % Metal Found                                                                           m.p.                                   ______________________________________                                        2    CoL.sub.6 (SO.sub.3 CF.sub.3).sub.2                                                       10.3        10.3      210° C.                         3    NiL.sub.6 (SO.sub.3 CF.sub.3).sub.2                                                        7.7         7.6      270° C.                         4    CuL.sub.4 (SO.sub.3 CF.sub.3).sub.2                                                       10.2        10.4      170° C.                         ______________________________________                                    

The above procedure, when carried out with the appropriate molar amountsof 2-ethyl-4-methyl imidazole provided the corresponding hexa- andtetra-2-ethyl-4-methyl imidazole metal trifluoromethanesulfonates ascolored precipitates. These precipitates were purified by washing withether and drying in vacuo.

The di-imidazole copper(II) perfluoromethane sulfonate was made byreacting precisely 0.25 mol of copper(II) perfluoromethanesulfonate withslightly more than 0.05 mol of imidazole in methanol. Copper(I)triimidazole perfluoromethanesulfonate was obtained by convertingcopper(I) chloride (see Brauer, Handbook of Preparative InorganicChemistry, Vol. II, 2nd Ed., Academic Press, New York, 1965, page 1005)to copper(I) perfluoromethanesulfonate with perfluoromethyl sulfonicacid and reacting with slightly more than three times the molar amountof imidazole.

EXAMPLE 5 Preparation of Copper(II) Tetraimidazole-Perfluoro(loweralkyl) Sulfonates

Approximately 0.5 mole of n-C₄ F₉ SO₃ H was reacted with slightly morethan 0.25 mol CuCO₃ suspended in water. The mixture was heated andfiltered from unreacted CuCO₃. The blue filtrate was slowly evaporatedon a hot plate. The salt obtained was a waxy solid.

This product (0.11 mol) was dissolved in CH₃ OH and imidazole (0.5 mol)was added slowly to the solution. Complexing with imidazole wasindicated by the deep blue color of the solution. The solution wasevaporated and addition of ether caused precipitation of a blue-violetpowder. The powder was found to have a melting point of about 190° C.

The same procedure was followed with n-C₈ F₁₇ SO₃ H to obtain theperfluorooctylsulfonate salt and from this the corresponding Cu(II)tetra-imidazole of perfluorooctylsulfonate coordination compound. Themelting point of this coordination compound was 210° C.

Both of the coordination compounds of this Example were found to belatent initiators capable of efficiently curing 10 g. samples of "Epon828" (Shell trade-mark) at temperatures above 100° C. The low exothermsevolved during these cures (for cylindrical castings about 20 mm. indiameter × 0.30 mm. in length plunged into a preheated 150° C. bath)resulted in peak exotherm temperatures of about 197° C; cf. Example 8.

EXAMPLE 6 Gel Time Determination

The varying degrees of latency of a curing agent of this invention weredemonstrated by determining the gel time (time required for the systemto "set") of a mixture of one gram of the product of Example 2 and 10grams of "Epon 828" (the polyglycidylether of Bisphenol A andepichlorohydrin), i.e. about 10 phr of curing agent. The resultingviscous curable system, which was pink in color, did not gel at roomtemperature. At 100° C., the gel time was 68 minutes, indicating thatsome latency remained even at this temperature, since a similar curablesystem containing free imidazole had a gel time of about 5 minutes at100° C. At 120° C., the gel time was still somewhat longer than that forfree imidazole: 9 minutes, 15 seconds, as against 2 minutes, 50 seconds.At 140°, 160°, and 180° C., however, the differences in gel time betweenthe curing agent of Example 2 and free imidazole were insignificant: 2minutes, 30 seconds, and 20 seconds, respectively, as against 1 minute,24 seconds, and 15 seconds.

EXAMPLE 7 Curing of Cycloaliphatic Epoxy Systems

One gram of each of the products of Examples 1-4 was mixed with 10 g."Ciba CY-179" (3,4-epoxycyclohexylmethyl3,4-epoxycyclohexanecarboxylate, i.e. E--CH₂ --O--CO--E, wherein E is3,4-epoxycyclohexyl) to provide the curable systems to be tested. Thus,in each case 10 phr of the mixture was the metalperfluoromethyl-sulfonate-imidazole. In each case the mixture wasunreactive and remained a viscous liquid at room temperature. Even at100° C., the metal perfluoromethylsulfonate-imidazoles remained latentfor about 55 minutes. However, when the mixtures were heated to 150° C.,the following results were observed: (L = imidazole)

                  TABLE I                                                         ______________________________________                                                                             Color of                                                                      System                                        Curing Agent                                                                              Cure Time    Barcol Prior to                                 Ex.  (10 wt. %)  (min.) at 150° C.                                                                   Hardness                                                                             Cure                                     ______________________________________                                        1    ZnL.sub.4 (SO.sub.3 CF.sub.3).sub.2                                                       10           84     Colorless                                2    CoL.sub.6 (SO.sub.3 CF.sub.3).sub.2                                                       10           84     Pink                                     3    NiL.sub.6 (SO.sub.3 CF.sub.3).sub.2                                                       10           86     Blue                                     4    CuL.sub.4 (SO.sub.3 CF.sub.3).sub.2                                                       10           85     Violet                                   ______________________________________                                    

A compound made according to Example 3 of DOS 1,904,641 did not cure the"CY-179", even at 150° C. A compound made from cobalt (II) chloride andimidazole in the imidazole-to-salt molar ratio of 6:1 also failed tocure the system at 150° C.

EXAMPLE 8 Exotherms During "Hot Entry" Cures

In the following "hot entry" cures, an oil bath was preheated to 150° C.and maintained at this temperature (±1° C.) with mechanical stirring.Ten gram samples of resin were poured at room temperature into a 20 ×100 mm thin-walled vessel, filling the vessel about one-third full. Anappropriate amount of curing agent, also at room temperature, wasrapidly mixed into the resin. The vessel was immediately placed in theoil bath, and a thermocouple was inserted into the middle of theresin/curing agent mixture, permitting accurate temperature measurementwith a potentiometer system. In each test of a sample, the sample wasbrought to 150° C. in approximately 60 seconds. In the following table,the "peak exotherm temperature" data refers to the highest temperatureobserved during curing. The peak exotherm above the cure temperature canbe derived from this data by subtracting 150° C.

The tabulated data of Table II clearly indicate that the initiators orcuring agents of this invention cure a wider variety of epoxides and/orcure epoxides with significantly lower exotherms than the curing agentsof DOS 1,904,641 or the metal perfluoroalkylsulfonates per se.

                  TABLE II                                                        ______________________________________                                        150° C. "Hot Entry" Cures (L = imidazole)                                       "Epon 828"    "Cy 179"                                                                  Peak              Peak                                                phr of  exotherm    phr of                                                                              exotherm                                            curing  temperature curing                                                                              tempera-                                 Compound   agent   (° C.)                                                                             agent ture (° C.)                       ______________________________________                                        Co(SO.sub.3 CF.sub.3).sub.2                                                              4       308          4    high                                                                          exotherm                                 Cu(SO.sub.3 CF.sub.3).sub.2                                                              4       308          4    high                                                                          exotherm                                 Ni(SO.sub.3 CF.sub.3).sub.2                                                              4       317          4    high                                                                          exotherm                                 Ni(CO.sub.2 CF.sub.3).sub.2                                                              4       *           *      *                                       Ni(CO.sub.2 CF.sub.3).sub.2                                                              10      *           *      *                                       L.sub.6 NiCl.sub.2                                                                       4       339         10    242                                      L.sub.4 CuCl.sub.2 **                                                                    4       300         *      *                                       L.sub.4 CuCl.sub.2                                                                       10      341         10    190                                      L.sub.6 CoCl.sub.2                                                                       4       324         *      *                                       L.sub.6 CoCl.sub.2                                                                       10      332         10    249                                      L.sub.6 Ni(CO.sub.2 CF.sub.3).sub.2                                                      4       325         10    245                                      L.sub.4 Cu(SO.sub.3 CF.sub. 3).sub.2                                                     10      209         10    190                                      L.sub.6 Co(SO.sub.3 CF.sub.3).sub.2                                                      10      203         10    207                                      L.sub.6 Ni(SO.sub.3 CF.sub.3).sub.2                                                      4       207         10    205                                      ______________________________________                                         *No reaction                                                                  **Example 3 of DOS 1,904,641                                             

What is claimed is:
 1. A compound of the formula

    ML.sub.n (O--SO.sub.2 --R.sub.f).sub.m

wherein M is a metal selected from Groups VIII, IB and IIB of thePeriodic Table, n is a coordination number of M ranging from 2 to 8,R_(f) is a perfluorinated alkyl or cycloalkyl group of 1 - 18 carbonatoms, m is the valence of M, and L is an imidazole of the formula##STR5## wherein R₁, R₂, and R₃ are the same or different and areselected from the group consisting of hydrogen, phenyl, methyl, benzyland ethyl.
 2. A compound according to claim 1, wherein R_(f) is aperfluorinated alkyl or cycloalkyl radical containing 1 - 8 carbonatoms.
 3. A compound according to claim 1 wherein n is a whole numberfrom 2 to 8, m is a whole number from 1 to 3, and M is selected from thegroup consisting of iron, cobalt, nickel, copper, cadmium, zinc, andsilver.
 4. A compound according to claim 1 selected from the groupconsisting of CuL₄ (SO₃ CF₃)₂, CuL₂ (SO₃ CF₃)₂, CuL₃ (SO₃ CF₃), CuL₄(SO₃ CF₂ CF₃)₂, CoL₆ (SO₃ CF₃)₂, CoL₆ (SO₃ CF₂ CF₃)₂, NiL₆ (SO₃ CF₃)₂,NiL₆ (SO₃ CF₂ CF₃)₂, NiL₆ (SO₃ CF₂ CF₂ CF₃)₂, CdL₂ (SO₃ CF₃)₂, and ZnL₆(SO₃ CF₃)₂ wherein L is as defined in claim
 11. 5. A compound accordingto claim 1 wherein L is imidazole.